Neither Silverlight 3 nor 4 Work on Outlook Folder Homepages
Starting in Silverlight 3, the algorithm that SL uses to determine the hosting URL of the SL control doesn’t work properly when the page is viewed as part of an Outlook Folder Home Page. This is actually a known issue and is being looked at for Silverlight 5 but was not discovered early enough to make it into Silverlight 4.
If you are developing a solution that includes the use of Outlook Folder Home Pages, I’d recommend you consider another option if you plan on doing any advanced things in your solution. Folder homepages are good for hosting plain ol’ HTML and an occasional Outlook View Control, but because of the security lock-downs that are added in each new version of Outlook, the flexibility you have for using various techniques inside OFHPs is limited. Depending on what you are trying to accomplish, you may want to consider using Form Regions and Ribbon customization or else simply providing a link to your high-complexity web page instead of providing the content inside Outlook itself.